In this article over at our pal mental_floss, Allen St. John wrote about
an intriguing research by Laurie Santos and Keith Chen of Yale University
to see if they can teach monkeys to spend money:

A video of one of these early experiments shows that when Felix,
the group’s alpha male, entered, he received a “wallet”
with 12 of those round aluminum tokens. Two student researchers, one
wearing a pink T-shirt, the other blue, stood on either side of that
3-foot cubic enclosure, each holding a different tray of food. The premise
at this stage was pretty basic: Felix could swap his tokens for food
with either of the two researchers. He didn’t seem to care much
about the students. But he did care profoundly about what the researchers
would sell him in exchange for that little metal token.

Felix and the others were cautious, observant shoppers. As the
video shows, Felix would head first to the researcher holding out pieces
of orange, examining them carefully; before leaving, he stopped to smell
them. He went to the other researcher and did exactly the same thing—looking,
sniffing, shopping. He then headed back to the first researcher and
handed over a token to complete the transaction. Oranges, please.

“When you watch it, it looks like they’re contemplating,
thinking about what they’re going to buy,” says Santos.
What separates these capuchins from the scores of animals who have been
trained to perform complex behaviors in exchange for food is the option
presented by that second researcher.

“The critical aspect of money is that it’s fungible.
It represents a choice,” explains Chen. “A coin is fundamentally
different than, say, pressing a lever.” Santos and Chen had not
only achieved their preliminary goal, they had made history: The monkeys
were using cash. The capuchins were now operating in a sphere where
humans had been dwelling alone.